Michael Porter Jr. strolled into the media interview room Thursday at the NBA draft combine wearing an all-white hoodie with just four words written across the front in small black letters: “No freedom to speak.”

Whether he was sending a message or simply liked the sweatshirt is unclear. But when the prized but perplexing forward from Missouri sat down, he had plenty to say in his 20-minute session. He started by declaring himself better than ever following back surgery last fall. He quickly pivoted to boasts of being the best.

“I played against all these guys,” Porter said of this year’s draft class. “They’re all great players. But I’m the best player in this draft. And I just can’t wait to show what I’m capable of.”

Before undergoing November back surgery, which sidelined him for all but three games of his lone collegiate season, Porter was pegged as a potential No. 1 overall pick. He could now slip to the Bulls at...